1. Field of the Invention
The present invention relates to an image pickup apparatus consisting of a single solid-state image pickup element for picking up an image via a color filter array, i.e., a single-plate color image pickup apparatus.
2. Related Background Art
As a conventional single-plate type color filter array, a so-called complementary mosaic type color filter array is popularly used and known in, e.g., video cameras. FIG. 7 shows an example of the color layout of this color filter array. This color layout has the following features. That is, the color layout shown in FIG. 7 matches well a so-called field reading method, and even when a sum of two pixels in the vertical direction is read out (in this case, such combinations of pixels are interlaced in units of fields), a relatively broad luminance signal range can be assured. In addition, since the complementary type color filter array is used, the sensitivity is high.
Such color filter array is normally applied to an interline CCD shown in FIG. 6. The interline CCD shown in FIG. 6 comprises photodiodes PD constituting pixels, vertical CCD shift registers V-CCD, and a horizontal CCD shift register H-CCD. Charge signals on desired photodiodes PD are simultaneously read out to the vertical CCD shift registers during a vertical blanking period. Thereafter, the readout charge signals are sequentially transferred inside the vertical CCD shift registers, and are output via the horizontal CCD shift register H-CCD and an output amplifier.
In recent years, multi-pixel CCDs (image pickup elements) have been developed. In particular, CCDs having pixels (1.5 to 2 million pixels) are available. In such situation, the layout of the complementary mosaic type color filter array suffers some problems.
More specifically, the problems are associated with a high-speed reading operation and a thin-out reading operation. When a multi-pixel sensor having, e.g., 1.6 million pixels is to be realized by an interline CCD shown in FIG. 6, if it is manufactured without using any special processes for the vertical and horizontal CCD shift registers, the time required for reading out all pixels is 1/15 sec about four times that (1/60 sec) in the field reading mode of an interline CCD having 0.4 million pixels. This means that a photometric operation (auto-focus (AF), auto-exposure (AE), and auto-white balance (AWB)) prior to a photographing operation of a so-called TTL (Through the lens) system requires much time. When such sensor is applied to a still video camera, or the like, the time required from the depression of the shutter until the start of main exposure is prolonged, and the user may lose a good shutter chance.
In view of this problem, a thin-out reading operation is performed using a substrate discharge operation popularly adopted in an interline CCD, thus shortening the reading time. This operation will be described below with reference to FIGS. 8A to 8C. Photoelectrically converted carriers are generated on photodiodes (to be abbreviated as PDs hereinafter) (FIG. 8A), and gates V1 (also serving as transfer gates) of a vertical CCD shift register (to be abbreviated as V-CCD hereinafter) are enabled to transfer C signal carriers to the gates V1 (FIG. 8B). A voltage is applied to a substrate to discharge M and G signal carriers in the substrate direction (FIG. 8C). Thereafter, the V-CCD is normally driven to transfer and output the carriers.
However, in this case, although a thin-out operation can be attained, the C signal carriers in FIG. 8C cannot be transferred by mixing (i.e., by decreasing the number of data), the transfer time still requires 1/15 sec upon field reading. In addition, since only the C and Y signal carriers of a chrominance signal are output, the output signal cannot be used in AWB control.
In order to solve this problem, a color filter array which has an 8-phase V-CCD, as shown in FIG. 9A and can form a chrominance signal even when a thin-out operation is performed is proposed. This color filter array will be described below with reference to the accompanying drawings. Photoelectrically converted carriers are generated in PDs (FIG. 9A). The color layout has a basic repetitive pattern constituted by 2.times.8 pixels, and the same pixel patterns appear every third lines in the vertical direction, as shown in FIG. 10, so that a chrominance signal can be generated even when a thin-out operation is performed later. Gates V1 and V3 are enabled to transfer C1, M1, C3, and G3 signal carriers to a V-CCD (FIG. 9B). A voltage is applied to the substrate to discharge C2, M2, C4, and G4 carriers (FIG. 9C). The potential of the V-CCD then lowers, C1 and M1 carriers and C3 and G3 carriers add up each other in the V-CCD to obtain C+M and C+G signal carriers, and these carriers are transferred (FIG. 9D).
According to this method, the number of carriers to be transferred can be halved (i.e., 1/30 sec). In addition, a chrominance signal can be output.
However, the image quality suffers when all the pixels are read out. In FIG. 11, the sampling carriers of luminance and chrominance signals in the color layout shown in FIG. 10 are plotted on the two-dimensional frequency plane. Note that 1/PH is the sampling frequency determined by the pixel interval in the horizontal direction, and 1/PV is the sampling frequency determined by the pixel interval in the horizontal direction. As can be seen from FIG. 11, the number of chrominance signal carriers on the 1/2PH axis is larger than that in the color layout of a complementary mosaic type filter array using a normal 2.times.4 pixel repetitive pattern, as shown in FIG. 12, and the amount of color moire increases as a whole. For this reason, when chrominance signal carriers are to be suppressed by, e.g., an optical LPF, an optical LPF having stronger cutoff characteristics is required, resulting in low resolution (especially in the horizontal direction).
As described above, in the above-mentioned prior art, when the color layout of the color filter array is changed to attain a high-speed thin-out reading operation, the image quality deteriorates upon reading out all pixels. Therefore, a thin-out reading operation that places an importance on image quality cannot be realized.